The present invention relates to a self adjusting hydraulic tappet, particularly suitable for heat engines in which the control of the valves takes place by means of direct control from the cams without the interposition of rockers; such a tappet is particularly suitable for motors the speed of rotation of which is very high.
As is known, the members which control the opening and closing of the valves in combustion engines, that is to say the cam and tappet, are subjected to wear in that they slide under pressure on one another, and in particular an element of the tappet, the cap, which is conveniently made of a material which wears more readily than that of the cam, becomes worn during operation thus causing a play which prevents the complete opening of the valve. Therefore, it is necessary periodically to effect adjustment with shims and, possibly, replace the cap. Moreover, the conditions of clearance between the cam and the valve vary in the various operating conditions of the engine, such clearance depending on the thermal expansion of the members of the kinematic chain interposed between the cam and the valve. There exist, however, mechanisms which adjust the clearance in a continuous and automatic manner as soon as it occurs: these are situated on the tappet and are substantially of hydraulic type.
A known hydraulic tappet comprises a first cup-shape body axially movable in a corresponding seat formed in the cylinder head of the engine and provided with a bottom wall which can be brought into contact with a cam of a cam shaft, and by a side wall, a second cup-shape body axially movable within the first and provided with a side wall and with an end wall which is able to define a chamber with the said walls of the first cup-shape body; in this chamber there is located a spring which can displace the second cup-shape body axially outwardly with respect to the first, and the end wall of the second cup-shape body is held in contact with the stem of a valve. The chamber thus defined is in communication, by means of ducts formed in the said cup-shape bodies, with a source of oil under pressure and along the said ducts there are positioned interception members.
Such interception members are constituted by a resiliently deformable flat plate of substantially annular form located within the said chamber: the inner peripheral circular edge region of the said plate is fixed to the end wall of the second cup-shape body, whilst its outer peripheral part is operable to close the end of the first mentioned duct, which opens into the said chamber. The fixing of the plate to the end wall of the second cup-shape body is normally achieved by means of the said spring located between the end walls of the two cup-shape bodies; for this purpose this spring is formed as a cup spring and has an inner peripheral edge which can abut on the said inner peripheral edge region, and an outer peripheral edge which can abut on the end wall of the said first cup-shape body.
The described tappet has several disadvantages. Above all, the oil seal of the said interception members is rather critical and therefore these allow a certain quantity of oil to escape from the said chamber with the consequence that these chambers tend to become empty in a short time after the engine has stopped, and to achieve a complete filling of these (a condition to which corresponds complete elimination of clearances) rather extended times are necessary. The seal obtained with such interception members is acceptable, then, only if the oil pressure in the chamber itself is rather high and corresponding to that which is achieved only during average running conditions of the engine.
Moreover the structure of the tappet is complex and therefore it is expensive and not very reliable. In fact, above all, the assembly constituted by the thin plate of the valve means and by the cup spring which holds this latter in the correct working position, includes two resilient members which must have very rigorous forms and dimensions, and which must be positioned in a very precise manner between two cup-shape bodies. In addition the seat against which this thin plate rests, which is formed in the end wall of the second cup-shape body, and the thin plate itself, must have a very high surface finish and strict tolerances on the form and dimensions in order to be able to cooperate in a correct manner with one another, and therefore these must be subjected to a lapping operation. Finally, the said thin plate, being cyclically deformed, is subjected to fatigue stresses which could bring about breakage of the thin plate itself.